The present invention relates to an embossing metal mold for transferring a fine concave and convex pattern of information to the surface of an optical disk substrate for recording/reproducing the information optically, and also to a manufacturing method thereof.
Since one bit which is a unit of information signals can be stored in a size of 1 .mu.m or less on an optical disk substrate, optical disks have been remarkably appreciated as high density and larger capacity media. As a substrate of such an optical disk, there are known three types in accordance with purposes, i.e., a substrate for an read-only optical disk, a substrate for an write once optical disk, and a substrate for an erasable optical disk. A concave and convex pattern corresponding to information, including pits corresponding to information or address pits and grooves for tracking a laser beam accurately, is formed on the optical disk substrate. Such optical disk substrates are mass-produced in such a manner that a concave and convex pattern corresponding to information is transferred thereto by a stamper, on which the concave and convex pattern corresponding to information is formed, in the injection mold method, the compression mold method or the 2P (Photo-Polymerization) method.
As disclosed in Japanese Patent Unexamined Publication No. 60-195749, a conventional stamper is manufactured in the following manner: a photo resist layer 3 is formed on a glass substrate 13; a concave and convex pattern corresponding to information is formed on the photo resist layer 3 as a result of exposure to laser light 4 for recording which is modulated by information signals, and development; a master disk thus produced is then baked at 140.degree. C.; a conductive layer 1 which is a metallic layer of Ni or the like is formed on the surface of the photo resist layer 3; an Ni electric-plating layer 2 is formed on the conductive layer 1 which functions as an electrode; the layers 1, 2 and 3 are peeled from the glass substrate 13; and the photo resist layer 3 is removed (see FIG. 2).
The thickness of the stamper used in the injection mold method and the compression mold method is concerned with the durability of the stamper with respect to a pressure of several tens tons/cm.sup.2, a heat cycle of 100.degree. C. to 300.degree. C. and so forth in the molding process. When the thickness of the stamper is small, its strength for pressure is insufficient, and the durability against the pressure is accordingly inadequate. When the thickness of the stamper is large, non-uniform internal strain is generated due to the heat cycle, thereby causing cracks in it. With such a stamper, therefore, it is impossible to produce a large number of substrates. Taking these matters into account, the stamper preferably has a thickness of about 300 .mu.m in general.
The Ni electric-plating process employed for manufacturing the stamper is a method in which a master disk is placed in an electrolytic solution of Ni sulfaminate acid so as to form an Ni electric-plating layer thereon according to the electrolysis method. An important factor in the Ni electric-plating process is that the plating treatment is carried out under such plating conditions that no stress is generated in the Ni electric-plating layer. It is because, if any stress exists, the Ni electric-plating layer will be bent. However, if the plating treatment is repeated many times under the same conditions which will not cause any stress, the electrolytic solution will change in quality, and some stress may exist in the Ni electric-plating layer. When the stress in the Ni electric-plating layer is larger than adhesive strength between substances existing between the glass substrate and the Ni electric-plating layer, peeling phenomena are observed between the Ni electric-plating layer and the conductive layer or between the conductive layer and the photo resist layer or between the photo resist layer and the glass substrate, or the like (in fact, most of peeling phenomena occur between the photo resist layer and the glass substrate). Then, the electrolytic solution enters peeled portions of these layers. This electrolytic solution contaminates the surface of the conductive layer and that of the Ni electric-plating layer, so that pits and projections are left on the surface of the stamper. These pits and projections, which are different from those corresponding to information signals, are defects and result in errors. With such a stamper, optical disk substrates of high quality can not be produced. In this case, another master disk must be manufactured and subjected to the Ni electric-plating process. Thus, there is a drawback that it is extremely difficult to form a stamper from a master disk without defects and reliably.
Moreover, in response to diversifications of industries, it has recently become more necessary to gain access to information covering a wider range, and accordingly, the number of kinds of information thus required has increased. Under the circumstances, there has also been an increasing demand for forming many kinds of information on optical disk substrates in short turn around time. Usually, one kind of information is formed on the surface of a stamper. Consequently, many stampers are required for manufacturing optical disk substrates of many kinds. However, it takes, for example, ten hours or so to produce a stamper because an Ni electric-plating layer is formed to have a thickness of 300 .mu.m in the electrolysis method. Therefore, it takes many days to prepare stampers of many kinds in the conventional method, thus resulting in another drawback that the recent demand can not be satisfied by the conventional method.
In other words, the conventional technique has a problem that it can not provide stampers as reliably and in as short turn around time as the developing industries demand.
In order to solve this problem, there is a stamper on which a pattern corresponding to information is formed by providing a photo resist layer, on which the pattern corresponding to information is formed, on a flat metallic plate which is polished with accuracy, and etching it with the photo resist layer serving as a mask on it, as disclosed in Japanese Patent Unexamined Publication No. 63-50937. However, polishing of the metallic plate can not be performed uniformly owing to impurities which the plate material may contain, and its remaining roughness will not be sufficiently reduced. Thus, there is induced a new problem that the S/N of read-out signal will not be improved. Further, an Ni substrate, of which surface is coated by Cr layer or the like by sputtering is disclosed in Japanese Patent Unexamined Publication No. 61-3339. However, it is not favorable because the roughness is greatly affected by the sputtering conditions and because the adhesive strength between the Ni substrate and the Cr layer is inadequate.